1. Field of the Invention
This invention relates to an apparatus and a method for processing hydrocarbon by-products that include heavy polynuclear aromatics (HPNA) for the purpose of increasing the production of lighter hydrocarbon fuels, such as gasoline and diesel, in conjunction with the operation of a hydrocracking process.
2. Description of Related Art
Hydrocracking processes are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling in the range of 370° C. to 520° C. in conventional hydrocracking units and boiling at 520° C. and above in the residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking processes typically improve the quality of the hydrocarbon feedstock by increasing the hydrogen to carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking processes has resulted in substantial development of process improvements and more active catalysts.
Hydrotreating and hydrocracking units generally include two principal zones, reaction and separation. Key parameters such as feedstock quality, product specification/processing objectives and catalysts typically determine the configuration of the reaction zone.
Mild hydrocracking or single stage once-through hydrocracking occurs at operating conditions that are more severe than hydrotreating processes, and less severe than conventional full pressure hydrocracking processes. This hydrocracking process is more cost effective, but typically results in lower product yields and quality. The mild hydrocracking process produces less mid-distillate products of a relatively lower quality as compared to conventional hydrocracking. Single or multiple catalysts systems can be used depending upon the feedstock processed and product specifications. Single stage hydrocracking is the simplest configuration, and are designed to maximize mid-distillate yield over a single or dual catalyst systems. Dual catalyst systems are used in a stacked-bed configuration or in two different reactors.
In a series-flow configuration, the entire hydrotreated/hydrocracked product stream from the first reactor, including light gases including C1-C4, H2S, NH3, and all remaining hydrocarbons, are sent to the second reactor. In two-stage configurations, the feedstock is refined by passing it over a hydrotreating catalyst bed in the first reactor. The effluents are passed to a fractionator column to separate the H2S, NH3, light gases (C1-C4), naphtha and diesel products boiling in the temperature range of 36-370° C. The hydrocarbons boiling above 370° C. are then passed to the second reactor.
The formation of heavy polynuclear aromatics (“HPNA”) is an undesirable side reaction that occurs in recycle hydrocrackers. The HPNA molecules form by dehydrogenation of larger hydro-aromatic molecules or cyclization of side chains onto existing HPNAs followed by dehydrogenation, which is favored as the reaction temperature increases. HPNA formation depends on many known factors including the type of feedstock, catalyst selection, process configuration, and operating conditions. Since HPNAs accumulate in the recycle system and then cause equipment fouling, HPNA formation must be controlled in the hydrocracking process.
Referring to FIG. 1, a conventional two-stage hydrocracking unit with recycling of unconverted fractions is illustrated in greater detail. A feedstock 11 is hydrotreated/hydrocracked in a first reactor 10 over a hydrotreating catalyst bed, usually comprising amorphous based catalyst(s), such as amorphous alumina or silica alumina substrates containing Ni/Mo, Ni/W or Co/Mo metals as the active phase. The first reactor effluents 12 are then fractionated, and the light fractions 21 containing H2S, NH3, C1-C4 gases, naphtha and diesel fractions boiling up to a nominal boiling point of 370° C. are separated. The hydrocarbon fraction 22 boiling above 370° C. are then sent to the second reactor 30 containing amorphous and/or zeolite based catalyst(s) having Ni/Mo or Ni/W metals as the active phase. The effluents 31 from the second reactor 30 are sent to the fractionator 20, in a combined stream 13 with effluent 12 from the first reactor 10, for separation of cracked components. The HPNA molecules form during the process and accumulate in the recycle stream. Therefore, in conventional two-stage hydrocracking processes, HPNAs must be rejected via a bleed stream 23 or processed separately to eliminate equipment fouling, or an effective catalyst must be used to eliminate the formation of HPNAs or to hydrogenate and hydrocrack these heavy molecules into smaller ones.
A number of references illustrate the use of multiple hydrocracking zones within an overall hydrocracking unit. The terminology “hydrocracking zone” is employed because hydrocracking units often contain several individual reactors. A hydrocracking zone can contain two or more reactors.
For instance, U.S. Pat. No. 3,240,694 illustrates a hydrocracking process in which a feed stream is directed into a fractionation column and divided into a light fraction and a heavy fraction. The light fraction passes through a hydrotreating zone and then into a first hydrocracking zone. The heavy fraction is passed into a second, separate hydrocracking zone, with the effluent being fractionated in a separate fractionation zone to yield a light product fraction, an intermediate fraction which is passed into the first hydrocracking zone, and a bottoms fraction which is recycled to the second hydrocracking zone.
A process is described in U.S. Pat. No. 6,113,775 for hydrocracking difficult to process feed streams at a reduced operating pressure by first dividing the feed stream into a light fraction and a smaller heavy fraction, and processing these fractions in separate reactors. The heavy fraction will normally contain the more difficult to process species and is processed in a once-thru reaction zone. The light fraction is processed in a higher conversion reaction zone which also receives the recycle stream produced in the product fractionation/recovery zone.
In U.S. Pat. No. 5,904,835, a process is described in which a hydrocracking feed stream is processed by passing a portion of the feed stream into each of two reaction zones, with the effluents of the two reaction zones being charged into a common separation and product recovery facility. Unconverted hydrocarbons recovered in the product recovery facility are passed into only one of the reaction zones. An objective of the process described in the '835 patent is to avoid full dual reaction trains.
A process described in U.S. Pat. No. 6,217,746 includes a two-stage hydrocracking process characterized by operation of a second hydrocracking zone at a reduced pressure, which is conducive to cracking the highly paraffinic effluent of the first hydrocracking zone. The process is also characterized by the passage of a partially compressed hydrogen makeup gas stream into the second hydrocracking zone followed by compressing the recovered gas from the second hydrocracking zone effluent to form make-up gas for the first stage hydrocracking zone. No recycle gas stream is provided for the second hydrocracking zone.
In U.S. Pat. No. 6,312,586, a process is described for upgrading heavy hydrocarbons to lighter distillates in a hydrocarbon conversion process which employs several parallel reaction zones each of which contain both hydrotreating and hydrocracking catalyst beds. The feed and liquid recycle from the bottom of the reaction zone is charged to the top of the uppermost catalyst bed. Hydrogen flow is countercurrent to the descending liquid, and products are removed overhead through vapor-liquid contactors. The flow of feed to one of the reaction zones is periodically stopped to allow sequential hydrogen regeneration of the catalysts within the reaction zone.
In U.S. Pat. No. 5,885,440 a hydrocracking process is described which employs a reactor containing a first stage hydrocracking catalyst containing hydrotreating and hydrocracking functions to reduce the recombinant mercaptan content and/or smoke point of a product recovered from the effluent of the hydrocracking reactor. The entire effluent of the hydrocracking reactor is first cooled and then passed through the hydrotreating catalyst reactor. The effluent of the hydrotreating catalyst reactor then continues throughout the customary cooling and separation steps employed in the product recovery system. The second stage effluent is also sent to the fractionator.
A process is described in U.S. Pat. No. 5,026,472 where high boiling hydrocarbons are upgraded to products including low aromatic content kerosene or jet fuel in a dual reaction zone process. Feeds such as gas oils are introduced to a hydrocracking reactor, with the effluent separated into vapor and liquid fractions. The vapor fraction is partially condensed to yield a liquid comprising kerosene/diesel boiling range hydrocarbons which is charged to a hydrogenation reactor. Liquid recovered from both reactors is passed to a common fractionator. Vapor remaining after the partial condensation is passed to a hydrogenation zone product separator to recover recycle hydrogen.
U.S. Pat. No. 4,713,167 discloses a multiple single-stage process for the conversion of heavy hydrocarbonaceous charge stock into a lower boiling distillate hydrocarbon product. Fresh charge stock and hydrogen are introduced into a first catalytic reaction zone for conversion of the feed. Hydrocracked product effluent from the first hydrocracking reaction zone that comprises a predetermined distillate fraction is passed with an cracked effluent from a second catalytic hydrocracking or accucracking reaction zone into a separation zone and separated into various hydrocarbon streams including a light hydrocarbon stream comprising the distillate product, a middle hydrocarbon stream and a heavy hydrocarbon stream. The middle hydrocarbon stream, which includes the distillate fraction and hydrogen, is introduced into the second catalytic hydrocracking or accucracking reaction zone for conversion into a lower boiling accucracked effluent stream comprising the distillate hydrocarbons boiling in the distillate product range.
In U.S. Pat. No. 4,950,384, a process is described in which a hydrocarbonaceous feedstock is hydrocracked by contacting the feedstock in a first reaction stage at elevated temperature and pressure in the presence of hydrogen with a first hydrocracking catalyst to obtain a first effluent, and separating from the first effluent a gaseous phase and a liquid phase at substantially the same temperature and pressure as the first reaction stage. The liquid phase of the first effluent is contacted in a second reaction stage at elevated temperature and pressure in the presence of hydrogen and a second hydrocracking catalyst to obtain a second effluent. At least one distillate fraction and a residual fraction are obtained from the combination of the gaseous phase and the second effluent by fractionation, and at least a portion of the residual fraction is recycled to a reaction stage.
Currently, there are other process solutions disclosed for removing HPNA from a hydrocracking unit. The solutions include installation of an adsorption column, recycling the hydrocracking unit bottoms stream to a vacuum distillation unit, recycling the stream to a solvent deasphalting unit, bleeding a portion of the hydrocracking unit bottoms stream and sending it to a fluid catalytic unit, and installing a separate hydrogenation unit for treating the entire recycle stream.
Other process options are costly and will require additional capital investment and manpower to operate. The option of recycling to the vacuum tower will decrease the vacuum distillation unit capacity and/or require an increase in its size. Removal, known as bleeding, of the recycle stream will cause a substantial yield loss, which will change the process economics substantially.
Therefore, it would be desirable to provide a hydrocracking method and apparatus that is more cost efficient and minimizes yield reduction.